Method and apparatus for uplink synchronization in wireless communications

ABSTRACT

A method and system for uplink (UL) synchronization of an uplink transmission from a plurality of wireless transmit/receive units (WTRUs) to a Node-B in a code division multiple access (CDMA) system. A Node-B receives a transmission including a UL synchronization (SYNC_UL) sequence from a WTRU. A sampler samples the transmission at a sampling rate which is higher than a chip rate. The samples are down-sampled and the SYNC_UL sequence is detected at a lower rate. A first significant path location of the detected SYNC_UL sequence is determined, and based on the first significant path location, a final significant path location is determined. The final significant path location is quantized and UpPCH POS  is transmitted to the WTRU to adjust a UL transmission timing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.10/973,134 filed on Oct. 26, 2004, which claims priority from U.S.Provisional patent application Ser. No. 60/518,141, filed Nov. 7, 2003,both of which are incorporated by reference as if fully set forthherein.

FIELD OF INVENTION

The present invention is related to wireless communications. Moreparticularly, the present invention is a method and system for uplink(UL) synchronization in wireless communications.

BACKGROUND

UL synchronization is a procedure that controls the transmit time of awireless transmit/receive unit (WTRU) in a cell such that ULtransmissions from a plurality of WTRUs arrive at a Node-B at the sametime. In general, each WTRU has a different propagation delay in a cellcovered by a Node-B. Therefore, if there is no synchronization mechanisminvolved, UL transmissions from WTRUs arrive at different times at theNode-B, which increases intra-cell interference. When UL transmissionsare aligned with each other, due to the orthogonality of spreadingcodes, the intra-cell interference is reduced significantly.

When a WTRU is powered on, the WTRU first establishes downlink (DL)synchronization with a cell using a DL synchronization (SYNC_DL)sequence transmitted in a DL pilot channel (DwPCH). Only after the WTRUhas established DL synchronization, can the WTRU start the ULsynchronization procedure. UL synchronization is achieved during arandom access procedure and therefore involves a UL pilot channel(UpPCH) and a physical random access channel (PRACH).

Although a WTRU establishes DL synchronization and can receive downlinksignals from the Node-B, the distance between the WTRU and the Node-B isstill uncertain. This leads to unsynchronized UL transmissions.Therefore, the first UL transmission from the WTRU is limited to aspecial time-slot, an uplink pilot time slot (UpPTS), in order to reduceinterference in traffic time slots.

In order to initiate a call, a WTRU first transmits a UL synchronization(SYNC_UL) sequence to a Node-B through a UpPCH. The available SYNC-ULsequences are broadcast through DwPCH. Under the current thirdgeneration partnership project (3GPP) standards, eight (8) SYNC-ULsequences are available to a Node-B. For initial transmission of theSYNC_UL sequence, open loop UL synchronization control is used for theUpPCH. The WTRU estimates the propagation delay Δt_(p) based upon thepath loss measured on the received primary common control physicalchannel (P-CCPCH) and/or DwPCH. However, this estimation of thepropagation delay is not accurate or reliable.

After the detection of the SYNC-UL sequence in a searching window, theNode-B evaluates the timing of UL transmissions of SYNC_UL sequences,and replies by sending adjustment information to the WTRU to modify itsUL transmission timing for the next transmission. This is done with afast physical access channel (FPACH) within the following four (4)sub-frames. After sending the FPACH, the UL synchronization isestablished. The UL synchronization procedure is also used for there-establishment of the UL synchronization when the UL is out ofsynchronization.

SUMMARY

The present invention is a method and system for UL synchronization ofUL wireless transmissions from a plurality of WTRUs to a Node-B. ANode-B receives a transmission including a SYNC_UL sequence from a WTRU.A sampler samples the transmission at a sampling rate which is higherthan a chip rate. The samples are down-sampled and the SYNC_UL sequenceis detected at a lower rate. A first significant path location of thedetected SYNC_UL sequence is determined and, based on the firstsignificant path location, a final significant path location isdetermined. The final significant path location is quantized and ULtiming information (UpPCH_(POS)) is transmitted to the WTRU to adjust aUL transmission timing of the WTRU.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an apparatus for uplink synchronization inaccordance with the present invention.

FIG. 2 is a flow diagram of a process for uplink synchronization inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention will be described with reference to the drawingfigures wherein like numerals represent like elements throughout.

Hereafter, the terminology “wireless transmit/receive unit” (WTRU)includes but is not limited to a user equipment, a mobile station, afixed or mobile subscriber unit, a pager, or any other type of devicecapable of operating in a wireless environment. When referred tohereafter, the terminology “Node-B” includes but is not limited to abase station, a site controller, an access point or any other type ofinterfacing device in a wireless environment.

The features of the present invention may be incorporated into anintegrated circuit (IC) or be configured in a circuit comprising amultitude of interconnecting components.

The preferred embodiment of the present invention will be described withreference to the current 3GPP standards. However, it should beunderstood that a specific sampling rate, processing rate, or anynumerical quantification which are set forth hereinafter are providedonly as an illustration, not as a limitation, of the preferredembodiment of the present invention, and any other sampling rate,processing rate or numerical quantification may be adopted inimplementing the teachings of the present invention.

FIG. 1 is a block diagram of an apparatus 100 for UL synchronization inaccordance with the present invention. The apparatus 100 comprises areceiver 102, a sampler 104, a down-sampler 106, a first correlator 108,a processing unit 110, a second correlator 112, and a quantizer 114.

A WTRU transmits a SYNC_UL sequence to a Node-B through the UpPCH beforetransmitting messages via a random access channel (RACH). The Node-Bmonitors the UpPCH for detecting SYNC_UL sequences transmitted fromWTRUs. Once the Node-B detects a SYNC_UL sequence, the Node-B transmitsa response to the detected SYNC_UL sequence via an FPACH. The responseincludes UL synchronization information, UpPCH_(POS), (which is a timingdifference of receipt of the SYNC_UL sequence from the WTRU with respectto a reference time at the Node-B). After the WTRU receives theresponse, the WTRU synchronizes UL transmissions in accordance with theUpPCH_(POS) included in the response message.

The receiver 102 receives transmissions via the UpPCH from the WTRU andforwards it to the sampler 104. The sampler 104 samples the transmissionat a rate which is substantially higher than a chip rate, 1/Tc. Underthe current 3GPP standard, UL synchronization is controlled at aresolution of Tc/8. Therefore, it is preferred for the sampler 104 tosample the transmission eight (8) times the chip rate, 8/Tc. The sampler104 outputs the sampled data to both the down-sampler 106 and the secondcorrelator 112.

The down-sampler 106 down-samples the sampled data at a lower rate,preferably at the chip rate, 1/Tc. In accordance with the preferredembodiment, the down-sampler 106 selects one out of eight (8) samples.The down-sampled samples are forwarded to the first correlator 108.

The first correlator 108 performs correlation of the down-sampledsamples with each of a plurality of SYNC_UL sequences. Under the current3GPP standards, eight (8) SYNC_UL sequences are assigned to each Node-B.Therefore, preferably, the first correlator 108 generates correlationresults with each of eight (8) SYNC_UL sequences, although the specificnumber of sequences is not required.

Initially, the WTRU determines UL transmission timing of a SYNC_ULsequence based on a measured propagation delay of the DwPCH and/orP-CCPCH. The initial delay measurement is used to restrict the searchsize of the first correlator 108. However, it is not very reliable.Therefore, the first correlator 108 needs to cover the whole cell size.For example, a cell radius of 11.5 km corresponds to approximately 49chips for a chip rate of 1.28 Mcps. Since the Node-B observes a two-waypropagation delay, the search window size for the first correlator 108should be greater than 98 chips for the worst case. The output of thefirst correlator 108 consists of lag positions and corresponding complexvalued correlation results. This output can be used as an initialchannel estimate.

The correlation results are output to the processing unit 110. Theprocessing unit 110 determines whether any SYNC_UL sequences have beendetected. In detecting a SYNC_UL sequence, the processing unit 110calculates the average signal power of each SYNC_UL sequence andcompares it to a noise threshold. If the average signal power of eachSYNC_UL sequence is above the noise threshold, the processing unit 110outputs to the second correlator 112 that the SYNC_UL sequence has beendetected. If the average signal power of each SYNC_UL sequence is notabove the noise threshold, the processing unit 110 performs no furtheractions, and the receiver continues to monitor the UpPCH. The processingunit 110 also determines an initial first significant path location(IFSPL) of the detected SYNC_UL sequence. The first significant path(FSP) is the first path (in time) in the channel impulse response abovethe noise threshold. The IFSPL is determined at a chip rate resolution.

Under the current 3GPP standard, the minimum step size required for ULsynchronization is Tc/8. Therefore, the Node-B is required to determineUL timing information, (UpPCH_(POS)), preferably at a resolution ofTc/8. The second correlator 112 receives samples sampled at eight (8)times the chip rate from the sampler 104, and performs correlation ofthe samples with the detected SYNC_UL sequence around the IFSPL. Sincethe second correlator 112 performs correlation only around the detectedIFSPL instead of the whole cell size, the correlation in the secondcorrelator 112 is performed much more quickly. The second correlator 112determines a final first significant path location (FFSPL) at aresolution of Tc/8.

The apparatus 100 may further, and optionally, comprise a quantizer 114to quantize the FFSPL. Under the current 3GPP standards, the UpPCH_(POS)is coded with 11 bits with a step size of Tc/8. The quantizer 114quantizes the FFSPL to the closest multiple of Tc/8. This quantizedvalue is converted to the UpPCH_(POS) for the detected SYNC_UL sequence,and transmitted to the WTRU for UL synchronization of the next ULtransmissions.

FIG. 2 is a flow diagram of a process 200 for uplink synchronization inaccordance with the present invention. The process 200 comprises twoprimary steps: first, to determine an IFSPL of a SYNC_UL sequence (step210), and then to zoom in around the IFSPL and perform a higherresolution search for an FFSPL (step 212). The process 200 commenceswhen UL transmissions are received via an UpPCH (step 202). Thetransmissions are sampled by a sampler at preferably eight (8) times thechip rate (step 204). The sampled data is down-sampled preferably to achip rate (step 206). The down-sampled data is correlated with SYNC_ULsequences, and it is determined whether any SYNC_UL sequence has beendetected (step 208). In detecting a SYNC_UL sequence, an average signalpower of each SYNC_UL sequence is calculated and compared to a noisethreshold. If the average signal power of each SYNC_UL sequence is abovethe noise threshold, a detection of the SYNC_UL sequence is declared,and if the average signal power of an SYNC_UL sequence is not above thenoise threshold, the process returns to step 202 to continue to monitorthe UpPCH. If a SYNC_UL sequence is detected, the process 200 proceedsto determine an IFSPL of the detected SYNC_UL sequence, preferably atthe chip rate (step 210). Once the IFSPL is determined, the FFSPL isdetermined using the sampled data sampled at eight (8) times the chiprate and the IFSPL (step 212). The FFSPL is quantized and converted tothe UpPCH_(POS). The Node-B transmits the UpPCH_(POS) to a WTRU to beused in adjustment of UL transmission timing for the next transmission(step 214).

While this invention has been particularly shown and described withreference to preferred embodiments, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the scope of the invention describedhereinabove.

1. A Node-B for synchronizing uplink (UL) transmission of a wirelesstransmit/receive unit (WTRU) in a code division multiple access (CDMA)wireless communication system, the Node-B comprising: a receiver forreceiving a transmission including a UL synchronization (SYNC_UL)sequence from a WTRU; a sampler for generating samples of thetransmission at a sampling rate; a down-sampler for down-sampling thesamples at a down-sampling rate; a first processing unit for detecting aSYNC_UL sequence in the transmission by processing down-sampled samplesat the down-sampling rate, and determining an initial first significantpath location of the detected SYNC_UL sequence; a second processing unitfor determining a final first significant path location of the SYNC_ULsequence in the transmission at the sampling rate using the detectedinitial first significant path location; and a transmitter fortransmitting a UL synchronization adjustment message generated based onthe final first significant path location to the WTRU to adjust a ULtransmission timing of the WTRU.
 2. The Node-B of claim 1 wherein thesampling rate is eight (8) times the chip rate of the WTRU transmission.3. The Node-B of claim 1 wherein the down-sampling rate is equal to thechip rate of the WTRU transmission.
 4. The Node-B of claim 1 wherein thesampling rate is inversely proportional to the resolution of the ULsynchronization sequence that is defined by the standards by which theWTRU is operating.
 5. The Node-B of claim 1 wherein detecting thepresence of a SYNC_UL sequence comprises generating correlation resultsfor each SYNC_UL sequence assigned to a Node-B.
 6. The Node-B of claim 1wherein a SYNC_UL sequence is detected when the average signal power ofa SYNC_UL sequence exceeds a predetermined noise threshold.
 7. TheNode-B of claim 1 wherein the final first significant path location isdetermined with a resolution of one eighth of the chip rate of the WTRUtransmission.
 8. The Node-B of claim 1 wherein the first finalsignificant path location is quantized for transmission.
 9. The Node-Bof claim 1 wherein the first final significant path location isquantized to the closest multiple of one eighth of the chip rate of theWTRU transmission.
 10. A code division multiple access (CDMA) wirelesscommunication system wherein at least one Node-B synchronizes the uplink(UL) transmissions of at least one wireless transmit/receive unit(WTRU), the system comprising: a. at least one WTRU comprising: i. atransmitter for transmitting an uplink synchronization (SYNC_UL)sequence through an UL pilot channel (UpPCH); ii. a receiver forreceiving an UL synchronization adjustment message; and iii. means foradjusting UL transmission timing in response to said UL synchronizationadjustment message; and b. at least one Node-B comprising: i. a receiverfor receiving a transmission including a SYNC_UL sequence from at leastone of said WTRUs; ii. a sampler for generating samples of thetransmission at a sampling rate; iii. a down-sampler for down-samplingthe samples at a down-sampling rate; iv. a first processing unit fordetecting a SYNC_UL sequence in the transmission by processingdown-sampled samples at the down-sampling rate, and determining aninitial first significant path location of the detected SYNC_ULsequence; v. a second processing unit for determining a final firstsignificant path location of the SYNC_UL sequence in the transmission atthe sampling rate using the detected initial first significant pathlocation; and vi. a transmitter for transmitting a UL synchronizationadjustment message generated based on the final first significant pathlocation to the at least one of said WTRUs to adjust a UL transmissiontiming of the at least one of said WTRUs.
 11. The system of claim 10wherein the sampling rate is eight (8) times the chip rate of the WTRUtransmission.
 12. The system of claim 10 wherein the down-sampling rateis equal to the chip rate of the WTRU transmission.
 13. The system ofclaim 10 wherein the sampling rate is inversely proportional to theresolution of the UL synchronization sequence that is defined by thestandards by which the WTRU is operating.
 14. The system of claim 10wherein detecting the presence of a SYNC_UL sequence comprisesgenerating correlation results for each SYNC_UL sequence assigned to aNode-B.
 15. The system of claim 10 wherein a SYNC_UL sequence isdetected when the average signal power of a SYNC_UL sequence exceeds apredetermined noise threshold.
 16. The system of claim 10 wherein thefinal first significant path location is determined with a resolution ofone eighth of the chip rate of the WTRU transmission.
 17. The system ofclaim 10 wherein the final first significant path location is quantizedfor transmission.
 18. The system of claim 10 wherein the final firstsignificant path location is quantized to the closest multiple of oneeighth of the chip rate of the WTRU transmission.